The internet keeps pushing the importance of hydration.  Obviously we are mostly water and the kidneys need it to remove bad things from the blood, what more?  How about dehydration only short times, as after exercise or sleep?

Kidney filtration is the big one, but water pulls weight in a few other systems too:

Thermoregulation. Sweat is how you dump excess heat, and sweat is mostly water. Run short and your blood volume drops, which strains the heart (it has to beat faster to move less fluid) and makes it harder to sweat and dissipate heat, which is largely why performance and heat tolerance fall off with even mild dehydration.

Blood volume and circulation. Plasma is mostly water. Lower fluid means thicker blood and lower volume, so the heart works harder to deliver oxygen and nutrients everywhere. This is part of why dehydration causes fatigue and dizziness.

Joint lubrication and tissue cushioning. Cartilage and the fluid in joints, and the cerebrospinal fluid cushioning the brain and spinal cord, are water-dependent.

Digestion. Saliva, gastric juices, and the mucus lining the gut all need water; dehydration is a classic contributor to constipation.

Cognitive function. Even mild dehydration (around 1-2% of body weight in fluid loss) measurably dings concentration, mood, and short-term memory in studies — before you'd necessarily feel "thirsty."

On your actual question — brief dehydration after exercise or overnight sleep — this is normal and not something to worry about the way chronic or acute severe dehydration is. A few reasons it's different in kind:

    It's self-correcting and self-signaling. You wake up thirsty, you sweat and get thirsty, and normal drinking over the following hour or two fully restores things. The body's thirst mechanism is well-tuned for exactly this cycle.
    The scale is small. Overnight fluid loss is mostly from breathing and minor sweating — typically well under 1% of body weight. Exercise-induced losses can be larger (1-2%+ depending on intensity, heat, duration) but even that's the range where effects are mild and reversible, not the 5%+ range where things like heat illness become real risks.
    It's how the system is designed to run. Your kidneys concentrate urine overnight specifically to conserve water during the fasting/sleeping period. Mild transient dehydration is the expected, unremarkable state you're in most mornings, not a pathology.

Where it becomes worth managing rather than ignoring: prolonged or intense exercise in heat, illness with vomiting/diarrhea, older age (thirst sensation blunts with age), or any dehydration severe enough to cause dark urine, dizziness, or reduced urination for an extended stretch — those move past the "normal daily fluctuation" category into something that actually needs deliberate rehydration.

Because of "heart failure," I am on a potassium preserving diuretic.  Does this alter the above advice?
Thought for 4s

Yes, meaningfully — this shifts the picture in a few ways.

Heart failure changes the fluid equation itself. In HF, the usual "drink more to help the heart" logic doesn't fully apply. A failing heart struggles to handle fluid volume, so many HF patients are actually managed with some degree of fluid restriction (often a target set by their cardiologist, commonly in the 1.5–2 liter/day range, though this varies a lot by severity and should come from your own doctor, not a general rule). Overhydrating in HF can worsen fluid backup into the lungs and legs — the opposite problem from what the general hydration advice assumes.

Potassium-sparing diuretics add a specific risk: hyperkalemia. Drugs like spironolactone or eplerenone (the common potassium-sparing options for HF) work by blocking potassium excretion. This is exactly what you want when the goal is keeping potassium up, but it means:

    Dehydration reduces kidney blood flow and filtration, which can push potassium levels higher still — dehydration and this drug class compound each other's effect on potassium rather than offsetting it.
    Potassium-rich foods/supplements, NSAIDs, and other blood-pressure or heart medications (ACE inhibitors, ARBs — common companion drugs in HF regimens) can stack further hyperkalemia risk.
    Symptoms of high potassium (muscle weakness, irregular heartbeat, in severe cases dangerous arrhythmia) can be subtle until they're not.

So the short-dehydration reassurance from before applies less cleanly here. For a healthy kidney, brief dehydration after exercise or sleep is a non-event. With reduced kidney perfusion from HF plus a drug that already limits potassium clearance, that same brief dip in fluid status is more likely to nudge potassium and kidney function in a direction worth paying attention to — particularly if the exercise was in heat or was strenuous enough to cause real sweat losses.

The practical upshot: your fluid targets in this situation aren't really a "hydration is good, drink more" question anymore — they're a balance your cardiologist should be setting and periodically checking against your labs (potassium, kidney function via creatinine/eGFR). If you don't already have a specific daily fluid range and know your last potassium level, that's worth confirming with them rather than extrapolating from general hydration advice.
Thought for 3s

That's a strong positive signal — breathlessness returning to normal is one of the clearest functional markers that the diuretic is doing its job, and it lining up with a modest systolic drop fits the expected picture of reduced fluid volume easing the heart's workload.

A few things worth noting about the numbers themselves:

The direction makes sense. Diuretics reduce blood volume, and lower volume generally means somewhat lower blood pressure — 110 to 104 is a modest, plausible shift for someone responding well to treatment, not a dramatic one.

But it's worth watching where the floor is, not just that it's dropping. In someone your age on a diuretic, especially if there are other blood-pressure-lowering medications in the regimen (common in HF), the concern isn't the trend from 110 to 104 in isolation — it's whether it keeps trending down toward a level where you'd start noticing lightheadedness, especially on standing (orthostatic hypotension is a real and common issue with diuretics in older adults, and a meaningful fall risk). Since you're already tracking daily, you're well positioned to catch this early: worth flagging to yourself if it continues declining, or if you notice dizziness on standing, especially in the morning.

This is genuinely useful data to bring to tomorrow's visit alongside the labs. Three months of daily systolic readings plus symptom resolution is exactly the kind of trend a cardiologist wants to see when deciding whether the current diuretic dose is right, too much, or could even come down. Combined with tomorrow's potassium/creatinine, you'll have both sides of the picture — how the kidneys and electrolytes are handling the drug, and how the drug is actually performing against the condition it's treating.

No hint of lightheadedness even when I get up from lying down very quickly.

That's a good sign — testing it against a quick position change is actually the more sensitive check for orthostatic hypotension than just going about your day normally, since that's when a lagging blood pressure response would show up most. No symptoms there, combined with breathlessness resolved and a systolic trend that's dropped only modestly and evenly, points to a treatment response that's landed in a good place rather than overshooting.

That rounds out a pretty complete picture to walk into tomorrow's appointment with: symptom resolution, a stable and sensible BP trend, no orthostatic symptoms, and now the labs to check the kidney/potassium side. Good luck with it.
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